feat(65): the clip mux — a third output on the same ffmpeg, drained off the frame path

Slice 2 of TASK 65. 602 green, 0 warnings. **Clips are still off**: no `ClipBuffer` is passed, so the
ffmpeg command line is byte-for-byte what it was before this commit, and a test asserts exactly that
(the original args are a strict prefix of the new ones).

- `FfmpegArgs` appends a fragmented-MP4 output to `pipe:1` only when `ClipBuffer` is set. One-second
  fragments (`-frag_duration 1000000`) rather than the 4 s GOP, so a clip can start inside the window
  instead of at the last keyframe. Stats already own stderr, so stdout was free.
- `Fmp4FragmentSplitter` turns that byte stream into whole fragments: `ftyp`+`moov` is the init
  segment, each `moof`+`mdat` is a fragment. It parses box headers itself, survives a box being cut
  anywhere across reads, and **resyncs instead of throwing** — a clip fault must never take the stream
  down.
- `FfmpegEncoder` reads it on its own task, never blocking the frame path. Any exception logs and
  disables clips for the session; the broadcast is untouched.
- `IEncoderProcess` gained `StandardOutputBytes` (the seam already had the text reader for probing).

Next slices: the replay layer + retention, then the Settings switch with disabled-but-remembered controls.

Services/Encoder/EncoderOptions.cs

@@ -50,6 +50,11 @@ public sealed class EncoderOptions
     /// <summary>Name of the mixer's audio pipe (default <see cref="DefaultAudioPipeName"/>).</summary>
     public string AudioPipeName { get; init; } = DefaultAudioPipeName;
 
+    /// <summary>TASK 65: when set, the encoder muxes a SECOND video output into this ring (fragmented
+    /// MP4 on stdout) for the replay buffer. Null = clips off, and the ffmpeg command line is exactly
+    /// what it was before the feature existed.</summary>
+    public Services.ClipRing? ClipBuffer { get; init; }
+
     /// <summary>GOP in frames = Fps × 4s — the YouTube keyframe ≤ 4s compliance bound.</summary>
     public int GopSize => Fps * 4;
 }

Services/Encoder/FfmpegArgs.cs

@@ -52,6 +52,19 @@ public static class FfmpegArgs
             args.AddRange(["-f", "mp4", options.RecordPath!]);
         }
 
+        // TASK 65: the replay buffer is a third mux of the same encoded frames. Fragmented MP4 on
+        // stdout (stats already own stderr) — one fragment per second, so a clip can start anywhere
+        // inside the chosen window instead of at the last 4-second keyframe.
+        if (options.ClipBuffer != null)
+        {
+            AddVideoTags(args, options, videoEncoder);
+            args.AddRange(["-c:a", "aac",
+                "-ar", options.AudioSampleRate.ToString(), "-ac", options.AudioChannels.ToString()]);
+            args.AddRange(["-movflags", "+frag_keyframe+empty_moov+default_base_moof"]);
+            args.AddRange(["-frag_duration", "1000000"]);
+            args.AddRange(["-f", "mp4", "pipe:1"]);
+        }
+
         return args;
     }
 

Services/Encoder/FfmpegEncoder.cs

@@ -118,6 +118,11 @@ public sealed class FfmpegEncoder : IFfmpegEncoder
         _stopRequested = false;
         _stderrLoop = RunStderrLoopAsync(process);
         StartDrainLoop(process);
+
+        // TASK 65: the clip mux is a side channel. It reads its own thread, never blocks the frame
+        // path, and a fault disables clips instead of touching the stream.
+        if (options.ClipBuffer is { } clipRing)
+            _ = Task.Run(() => ReadClipOutputAsync(process, clipRing));
     }
 
     /// <summary>
@@ -239,6 +244,24 @@ public sealed class FfmpegEncoder : IFfmpegEncoder
     /// ArrayPool may return a larger buffer), returned to the pool after the write
     /// copies into the pipe, and stdin is closed (EOF) once the queue drains.
     /// </summary>
+    private static async Task ReadClipOutputAsync(IEncoderProcess process, Services.ClipRing ring)
+    {
+        var splitter = new Fmp4FragmentSplitter(ring);
+        var buffer = new byte[64 * 1024];
+        try
+        {
+            var stdout = process.StandardOutputBytes;
+            int read;
+            while ((read = await stdout.ReadAsync(buffer).ConfigureAwait(false)) > 0)
+                splitter.Write(buffer.AsSpan(0, read));
+        }
+        catch (Exception ex)
+        {
+            // Clips stop; the broadcast does not.
+            AppLog.Write(ex, "Clip buffer reader failed — replay clips disabled for this session");
+        }
+    }
+
     private void StartDrainLoop(IEncoderProcess process)
     {
         _drainTask = Task.Run(async () =>

Services/Encoder/FfmpegEncoderProcess.cs

@@ -21,6 +21,8 @@ public sealed class FfmpegEncoderProcess : IEncoderProcess
     }
 
     public Stream StandardInput => _process.StandardInput.BaseStream;
+
+    public Stream StandardOutputBytes => _process.StandardOutput.BaseStream;
     public TextReader StandardOutput => _process.StandardOutput;
     public TextReader StandardError => _process.StandardError;
 

Services/Encoder/Fmp4FragmentSplitter.cs

@@ -0,0 +1,102 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+namespace ytLive.Services.Encoder;
+
+/// <summary>
+/// TASK 65: turns the fragmented-MP4 byte stream ffmpeg writes to stdout into a playable ring.
+///
+/// <para>MP4 boxes are length-prefixed, so this needs no muxer cooperation: <c>ftyp</c>+<c>moov</c> are
+/// the initialisation segment, and each <c>moof</c>+<c>mdat</c> pair is one fragment. The ring only ever
+/// sees whole fragments, which is what makes a snapshot playable and a trim safe.</para>
+///
+/// <para>Deliberately forgiving: an unparseable header clears the buffer and waits for the next one. A
+/// clip that fails must never take the encoder down with it — the stream is running through the same
+/// process.</para>
+/// </summary>
+public sealed class Fmp4FragmentSplitter
+{
+    private readonly ClipRing _ring;
+    private readonly List<byte> _pending = new(capacity: 64 * 1024);
+    private readonly MemoryStream _init = new();
+    private readonly MemoryStream _fragment = new();
+    private bool _inFragment;
+
+    public Fmp4FragmentSplitter(ClipRing ring) => _ring = ring;
+
+    public void Write(ReadOnlySpan<byte> data)
+    {
+        foreach (var b in data) _pending.Add(b);
+        Drain();
+    }
+
+    private void Drain()
+    {
+        while (true)
+        {
+            if (_pending.Count < 8) return;
+
+            long size = ((long)_pending[0] << 24) | ((long)_pending[1] << 16) | ((long)_pending[2] << 8) | _pending[3];
+            var type = Encoding.ASCII.GetString(_pending.GetRange(4, 4).ToArray());
+            var header = 8;
+
+            if (size == 1)
+            {
+                if (_pending.Count < 16) return;
+                size = 0;
+                for (var i = 8; i < 16; i++) size = (size << 8) | _pending[i];
+                header = 16;
+            }
+
+            if (size < header || size > int.MaxValue)
+            {
+                // Not a box boundary any more; drop what we have and resync on the next write.
+                _pending.Clear();
+                return;
+            }
+
+            if (_pending.Count < size) return;
+
+            var box = _pending.GetRange(0, (int)size).ToArray();
+            _pending.RemoveRange(0, (int)size);
+            Route(type, box);
+        }
+    }
+
+    private void Route(string type, byte[] box)
+    {
+        switch (type)
+        {
+            case "ftyp":
+                _init.Write(box);
+                break;
+            case "moov":
+                _init.Write(box);
+                _ring.SetInit(_init.ToArray());     // init is complete once moov lands
+                break;
+            case "moof":
+                FlushFragment();
+                _fragment.Write(box);
+                _inFragment = true;
+                break;
+            case "mdat":
+                if (!_inFragment) return;           // mdat without a moof is not replayable
+                _fragment.Write(box);
+                FlushFragment();
+                break;
+            default:
+                if (_inFragment) _fragment.Write(box);
+                break;
+        }
+    }
+
+    private void FlushFragment()
+    {
+        if (_inFragment && _fragment.Length > 0)
+            _ring.Append(_fragment.ToArray());
+        _fragment.SetLength(0);
+        _inFragment = false;
+    }
+}

Services/Encoder/IEncoderProcess.cs

@@ -17,6 +17,10 @@ public interface IEncoderProcess : IDisposable
     Stream StandardInput { get; }
 
     TextReader StandardOutput { get; }
+
+    /// <summary>TASK 65: the same stdout as raw bytes — the replay-buffer mux writes fragmented MP4
+    /// here while <c>-stats</c> keeps stderr for itself.</summary>
+    Stream StandardOutputBytes { get; }
     TextReader StandardError { get; }
 
     bool HasExited { get; }